Appliance defrosting system and switch means

ABSTRACT

A defrosting system for an appliance having circulating airflow over cooling coils and means for supplying coolant to the coils, selectively operable means for defrosting the appliance including switch means responsive to a reduction of airflow within the appliance because of frost buildup on the cooling coils to initiate the defrost cycle, and additional switch means responsive to the temperature within the appliance to discontinue the defrosting operation and place the appliance back in condition for normal operation. Switch means are provided for use in the circuit which are uniquely responsive to airflow and which have other structural features uniquely adapting them to their application in a defrosting system.

United States Patent [191 Grover [451 July 30, 1974 Philip D. Grover, 3507 Glenwood Ave., Two Rivers, Wis. 54241 Filed: Aug. 7, 1972 Appl. No.: 278,524

[76] Inventor:

[56] References Cited UNITED STATES PATENTS 12/1960 Von Arb 62/140 3/196] Pietsch 62/140 3/1967 .lacobus 62/140 2/1972 Winkler 62/140 Primary Examiner-Meyer Perlin Attorney, Agent, or Firm-Hofgren, Wegner, Allen, Stellman & McCord v 57 ABSTRACT A defrosting system for an appliance having circulating airflow over cooling coils and means for supplying coolant to the coils, selectively operable means for defrosting the appliance including switch means responsive to a reduction of airflow within the appliance because of frost buildup on the cooling coils to initiate the defrost cycle, and additional switch means responsive to the temperature within the appliance to discon tinue the defrosting operation and place the appliance back in condition for normal operation. Switch means are provided for use in the circuit which are uniquely responsive to airflow and which have other structural features uniquely adapting them to their application in a defrosting system.

. v 9 Claims, 7 Drawing Figures 4'21 J/ M Z mamsomown 3326.103

SHEET 2 0F 2 neosr BU/L pu COMPRESSOR OFF com eEssok 01v i/ \l i [C E 50\ MEL TING TEMPE/Q4 TU QE APPLIANCE DEFROSTING SYSTEM AND'SWITCH MEANS BACKGROUND OF THE INVENTION This invention pertains to defrosting systems for an appliance and to switch means usable in such a system.

Various appliances including refrigerators, air conditioners and dehumidifiers have cooling coils provided with a coolant and with means providing a circulating airflow over the coils. Such appliances all have problems with the condensation of moisture on the cooling coils which builds up as frost or ice. This layer of frost or ice interferes with the heat absorption process between the coils and the circulating air and the general space within which the cooling coils are located and the temperature cannot be maintained at the desired level as well as causing inefficient operation. Such devices, therefore, have provision for defrosting and, with modern refrigerators, this is normally accomplished by use of a timing device that periodically switches the cir cuitry for the refrigerator from a refrigerating mode of operation to a defrosting mode of operation. The timing is set for an estimated frequency of defrosting operation and the time duration thereof. This is only an approximate operation because of the variations in the humidity of air as well as the number of times that the refrigerator is opened so that the timer-controlled defrosting operation does not directly correlate with the need for defrosting.

The timer-controlled defrost operation provides a fixed frequency of defrosting as well as a fixed duration of defrosting operation. This can result in an extreme condition of frost buildup where the coolant supply means, such as a compressor, runs continuously trying to reduce the temperature, without succeeding. Such timer-controlled defrosting operations include mechanical parts which are subject to wear and, therefore, the timer 'control has a limited life resulting in expensive service and repair of the appliance.

SUMMARY The invention embodied herein relates toa defrosting system for an appliance, such as a refrigerator, air conditioner or dehumidifier, which provides the defrosting operation when required and for the necessary length of time.

The defrosting system is responsive to the airflow through the appliance to detect a predetermined reduction in airflow which indicates a buildup of frost or ice on the cooling coils and the need for defrosting. The system further detects the removal of the frost or ice by a rise in the temperature level within the appliance and terminates the defrost operation.

A primary object of this invention is to provide a new and improved defrosting system for an appliance having coolant means and air circulating means that defrosts the appliance only when defrosting is required and for a length of time required to complete the defrosting.

Another object of the invention is to provide a defrosting system for an appliance having a circulating air flow over cooling coils and means for supplying coolant to the coils comprising, selectively-operable means for defrosting the appliance, means responsive to reduced airflow because of frost buildup on the cooling coils to actuate said selectively-operable means, and means responsive to a predeterminedtemperature level in the appliance for discontinuing operationof the defrosting means.

Still another object of the invention is to provide a defrosting system,.as defined in the preceding paragraph, wherein the means responsive to reduced airflow includes a switch with a switch-operating member shiftable upon change in temperature thereof and more particularly wherein said switch-operating member is electrically heated and normally maintained below a temperature at which it will shift by a normal volume of circulating airflow and, more specifically, wherein said switch-operating member is of bimetal.

Still another object of the invention is to provide a defrosting system as defined in the preceding paragraphs wherein the means responsive to a predetermined temperature level in the appliance for discontinuing operation of the defrosting means includes a temperature-responsive switch having a normally closed contact in series with the airflow-responsive switch and switchable to a second position at a higher temperature level in the appliance to establish normal operation of the appliance and with said temperatureresponsive switch returning to initial position responsive to a lower temperature of the appliance which indicates normal operation may commence.

Other objects of the invention are to provide switch means particularly adapted for use inthe circuit for the defrosting system and which are of a new and novel construction providing for use thereof in other circuits, including a snap switch having a knife-edge hinging action between the tension and compression members thereof to insure precision actuation and repeatability in the action.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a central vertical section from front to rear of an appliance, such as a refrigerator, embodying my invention;

FIG. 2 is a schematic drawing of the electrical circuit and components thereof as used in my defrosting system;

FIG. 3 is a time diagram of the operation of the appliance with my defrosting system incorporated therein;

FIG. 4 is a vertical central section of the override switch which is responsive to two different temperature levels in the appliance;

FIG. 5 is a perspective view of the airflow-responsive switch which detects the need for a defrost cycle of operation;

FIG. 6A is a perspective view of the snap switch element usable in the switches of FIGS. 4 and 5 and;

FIG. 6B is a view, similar to 6A, of a modification of the snap switch element.

DESCRIPTION OF THE PREFERRED EMBODIMENT As stated previously, the defrosting system is provided for use with appliances having cooled circulating airflow therethrough and the-system is shown in association with a refrigerator, although it may be used with other appliances, such 'as air conditioners and dehumidifiers. As shown particularly in FIG. 1, the refrigerator has a cabinet, indicated generally at 10, and has access doors 11 and 12 at the front thereof which provide access to a refrigerating section 15 and a deep construction has a pair of wall sections 17 and 18 which have horizontal and vertical components together defining a hollow wall through which air can flow, as indicated by the arrow, and with the airflow being induced by a fan 20' driven by a motor 21. To facilitate airflow through the entire appliance, the wall sections 17 and 18 are each apertured, as indicated at 22 and 23, respectively, and with an additional wall 25 defining, with the interior surface of the back wall of the refrigerator, an air passage terminating in openings 26 and 27 at the rear of the refrigerating section 15. Suitable panels 30 and 31 are shaped and positioned relative to the fan 20 to permit airflow while covering the fan and preventing contact therewith.

A plurality of cooling coils, indicated generally at 40 and 41, are positioned within the hollow dividing wall which conventionally cause cooling by expansion of compressed gas which removes heat from the surfaces in contact therewith as well as the circulating air travelling thereby. These cooling or expansion coils 40 and 41 are conventionally supplied with the compressed gas by suitable pipe connection from equipment located within the base of the refrigerator, including a motor compressor unit, indicated generally at 44, and a condenser, indicated generally at 45. The gases, after expansion in the cooling or expansion coils 40 and 41, are returned to the motor compressor 44 where the gases are compressed. The compressed gases then flow to the condenser 45 wherein the temperature of the condensed gas is reduced by airflow which enters through a front opening 46 in the base and exhausts through a rear opening 47. The airflow may be induced by a motor driven fan 48, although use of the fan is optional. The high pressure gas leaving the condenser 45 then passes to an expansion valve (not shown) where the gas is emitted through a small orifice and expands with a large drop in temperature into the cooling coils and 41.

Water resulting from melting of frost flows to a vertical pipe 49 and collects on a tray 49a for evaporation by heat created by the compressor 44.

All of the structure previously described is that which is conventional in a refrigerator; however, variations may be found in units of different manufacturers.

My defrosting system for the appliance is shown generally in FIGS. 1 and 2 and, for simplification, in FIG. 2, the electrical circuit is shown by single line. The circuit is in a line having opposite ends at 50 and 51 and includes a master on-off switch 52 which is closed when the refrigerator is to operate. A thermostatic switch 53, as conventionally known, causes operation of the components of a refrigerator to maintain a desired degree of coldness in the refrigerator. As shown in FIG. 2, the fan motor 21 for inducing circulating airflow through the refrigerator, the motor compressor 44 and fan 48 are in this circuit and normally operate when the thermostatic switch 53 is closed. Some refrigerators have provision for operating the fan motor 21 at other times, such as when the door of the refrigerator is opened.

The circuit for the defrosting system includes two primary switches, with the first of these switches being a single pole double throw airflow-responsive switch, indicated generally at 60, and the second of these switches being a single pole double throw override switch, indicated generally at 61. These two switches are shown in FIGS. Sand 4, respectively. The switch 60 is positioned in the hollow wall in the flow of the circulating'air past the cooling coils 40 and 41 and, thus, subjected to the velocity of airflow. The override switch 61 may be located in a suitable position within the refrigerator to determine the actual temperature existing at the desired location. I

In normal operation of the refrigerator, the switch arm 62 of the airflow switch 60 and the switch arm 63 of the override switch 61 are in a normally-closed position whereby the circuit is closed to the fan motor 21, motor compressor 44 and fan motor 48, subject to the position of the thermostatic switch 53. When the latter switch closes for operation of the refrigerator, the various components will operate until the thermostatic switch is satisfied by a desired reduction of temperature in the refrigerator.

The airflow-responsive switch 60 is normally in the condition shown in FIG. 2 when there is normal airflow past the cooling coils 40 and 41. The construction thereof is such that a calculated reduction in airflow due to buildup of frost or ice on the cooling coils 40 and 41 will cause movement of the switch arm 62 from its normally closed contact with contact 64 to the broken line position where it engages contact 65. This, then, places a line 66 circuit and, since there is not adequate cooling in the refrigerator, the thermostatic switch 53 is also closed, whereby a heater 67 having one or more sections positioned adjacent the cooling coils is energized to melt the frost or ice off the coils. The movement of the switch arm 62 has disconnected the fan motor 21, motor compressor 44 and fan motor 48 from the circuit.

The operation of the heater 67 continues until the override switch 61 senses an increase in temperature in the refrigerator to a predetermined level, at which time the switch arm 63, thereof, will move the engagement with a contact 70 into the broken line position shown in engagement with a contact 71. This removes the airflow-responsive switch 60 from the circuit and, again, places the fan motor 21, motor compressor 44 and fan motor 48 in the circuit. This causes the components to operate to reduce the temperature within the refrigerator and when a predetermined lower temperature level is sensed by the override switch 61 the switch arm 63 returns to its position in engagement with contact 70. Prior to this time, the airflow responsive switch 60 will have returned to the position wherein its switch arm 62 is in engagement with the contact 64 whereby a defrost cycle can occur when required.

A graph of the operation of the refrigerator with my defrost system is shown in FIG. 3 with time being represented along the abscissa and with normal operation within the refrigerator being shown to the left of the line and with the fluctuating temperature within the deep freeze unit 16 being shown and being beneath the 32 F line. In normal usage, there will be a slow frost buildup on the cooling coils 40 and 41 so that the slope of the line 81 representing temperature may decrease slightly, indicating a longer cycle of operation. The line 80 on the graph represents the point where the frost the heat of fusion during the melting of the ice, there is a length of time, represented by a part 83 of the temperature line, where the temperature rises slowly. After the melting of the ice, the temperature rises rapidly, as indicted at 84. At a preset temperature, indicated at 85, the override switch 61 moves to the position wherein the contact 71 is closed, which causes the refrigeration cycle to start with a reduction in temperature as indicated by part 86 of the line.

The override switch 61 senses a temperature reduction which, at a point approximately identified at 87, causes resetting of the override switch 61 to the normally-closed position wherein the refrigerator is back in normal operation, subject to the thermostatic switch 53. Prior to reaching the temperature indicated at point 87, the airflow-responsive switch 60 will have switched back to a position wherein the contact 64 is engaged by the contact arm 62 so that the unit is conditioned for going into defrost operation, except for the fact that the override switch 61 still has not switched back to its normally-closed position. As indicated at the right-hand end of the time chart, the refrigerator again can be in normal operation.

Referring first to the airflow-responsive switch 60, it is shown particularly in FIG. 5. This switch has a mounting base 90 with a mounting block 91 mounting a pair of conducting blades 92 and 93 for the switch contacts 64 and 65. The switch arm 62 is in the form of a snap switch to be more particularly described subsequently and is shown in its normally-closed position to which it is biased by its construction, with a contact 94 in engagement with the contact 64. The snap switch is mounted to thebase 90 by a non-conducting block 95 and the snap switch is made of electrically conductive material whereby it is in circuit to have electricity flow to either the contacts 64 or 65.

The airflow-responsive switch 60 is made responsive to airflow by the use of a bimetal strip 100 mounted at one end for flexing in response to the temperature which it encounters. This bimetal strip 100 is electrically-heated by means of a heater 101, shown in the circuit of FIG. 2. The orientation of the bimental strip 100, as shown in FIG. 5, is when it is heated by the heater 101 and is cooled by the normal flow of air within the refrigerator. When there is a normal velocity of airflow, sufflcient heat is removed from the bimetal to prevent it flexing downwardly from the position shown in FIG. 5. When there is a reduction in airflow, due to buildup of frost on the cooling coils 40 and 41, there will be less removal of heat from the bimental strip 100 with the result that it will flex downwardly and cause a screw 102 carried at the free end thereof to press downwardly on an actuating end of the snap switch 62 to cause it to snap overcenter whereby the contact 94 leaves contact 64 and engages contact 65. The snap switch 62 is biased upwardly and, once the bimental 100 returns to the position shown in FIG. 5, the snap switch returns to the position shown. Characteristics of the bimetal strip 100 and the heater 101 can be selected to provide proper response, dependent upon the airflow velocity in the refrigerator with fine adjustments being provided by adjustment of the screw 102 which is threadably mounted in the free end of the bimetal strip 100. The bimetal strip is composed of a low coefficient of thermal expansion material laminated to a high coefficient of thermal expansion material which is normally fabricated to be substantially flat at ambient temperature, as shown in the position of FIG. 5. As an increase in temperature occurs it will, as shown in FIG. 5, bend downwardly.

In the'switch of FIG. 5 and with the heater 101 in operation, the bimetal strip would normally be deflected downwardly; however, in use airflow travels across the bimetal strip and provides an action which parallels the widely-recognized chill factor as applied to the human body. The normal amount of airflow will take sufflcient heat from the bimetal strip so that it will not move downwardly to operate the switch. However upon a reduction in airflow sufflcient to indicate that a defrost operation must occur, then the bimetal strip'100 retains sufflcient heat to flex and operate the switch.

The override switch 61 is in the circuit to assure proper operation. For example, when a refrigerator is initially placed into operation, the circulating airflow i is not sufficiently cold to remove heat from the airflowsensing switch 100 and, therefore, the switch 60 would tend to place the circuit in defrost operation. The override switch 61 prevents this type of operation.

The override switch 61 is basically a temperaturesensing switch which is set to operate at two different temperature levels. This switch is constructed to have a relatively large differential between the two positions of operation at different temperatures.

The override switch 61 has a base of dielectric material with an assembled configuration of parts which provides mountings for the contacts 70 and 71 including conducting blades 111 and 112, interfitted with insulting blocks and with the configuration also mounting a bimetal strip 115 and a leaf spring member 116. The entire assembly is held together by insulated mounting bolts 117. The bimetal strip 115 is cantilevermounted at one end and carries an adjusting screw which may be adjusted to a desired operating point where the lower end thereof engages an operating end of the switch arm 63. The switch arm 63 is a snap switch element. As the temperature rises in the appliance, the bimetal strip 115 will deflect downwardly to snap the switch arm 63 to an oppositely-concave configuration, as shown in broken line in FIG. 4.

The override switch 61 also includes provision for returning the switch arm 63 to the full line position when a lower temperature is reached, as indicated at point 87 on the graph of FIG. 3. This structure includes a U- shaped member of nonconducting material connected to the leaf spring 116 and having an adjusting screw 126 threaded thereinto which engages the upper side of the bimetal strip 115. Thus as the bimetal strip 1 15 returns upwardly, the motion thereof is transmitted through the adjusting screw 126 to the U-shaped arm 125 and with a point 127 engaging the snap switch to return it to the full line position. The switch arm 63 is fastened to the base 110 and to an electrical connection by a member 130. The base 110 is shaped to coact with a deformed portion 131 of the switch arm whereby the switch arm is retained against pivoting and provides spring properties for the switch arm in compression along its lateral axis.

The snap switch element, as used for the switch arms 62 and 63, is shown more particularly in FIG. 6A wherein a body formed of metal having spring properties has a longitudinal opening defining a compression member and a generally parallel-extending tension member 141 and with a transverse plane portion 142 forming an actuating end. The U-shaped deformed portion or notch 131 in the compression member 140 forms a mounting plane portion having the opening 143 and a springing portion carrying the contact 94. The contact 94 is mounted on the approximate center line of the snap element 140. and at an end thereof and with the same end of the snap element having downwardlyangled portion 145 with a slot therein forming a knife edge 146 at or close to the plane surface of the snap element. The tension member 141 has a bent portion 147 at an end thereof engageable within the slot in part 145 and engageable with the knife edge 146 thereof as shown particularly in FIG. 5. The parts are constructed to have the tension member 141 in tension and the compression part 140 in compression. This results in a slight normal upward bow in the snap switch element.

The notch 131 has walls that are essentially vertical and a certain amount of lateral twist will occur as the snap element is actuated with this lateral springing serving to prevent any snap action caused by buckling of the planar construction of the compression member 140. This construction eliminates a double snap and provides a crisper, more precise action, since the tension member 141 does not have to bend as its plane of tension moves relative to the angled planes of the actuator end 142 and the opposite end carrying the contact 94. This construction also has a bilateral dissymmetry causing a twisting and lateral sliding motion of the contacts which helps to prevent sticking and gives a wiping action which helps keep the contacts clean.

In use in the airflow-sensing switch of FIG. 5, the snap element is only stable in one position, while as used in the override switch of FIG. 4, the snap element is to be bi-stable. The stability of the snap element in either flexed position, as used in FIG. 4, is provided by provision of an upward bend (not shown) but indicated by the broken line 150 in FIG. 6A. Thus, the same snap element can be used in both the switches 60 and 61 and with the upward bend 150 provided in the compression member to provide stable positioning in two positions for use of the snap element in the override switch of FIG. 4.

An alternate construction for the snap element is shown in FIG. 68 wherein the same parts as shown in FIG. 6A are given the same reference numerals with a prime affixed thereto. In this embodiment, the tension member is formed by a separate strip 160 having a connection at one end by a knife edge connection, as in FIG. 6A and having a similar connection to the actuating end of the snap element provided by a downturned part 161 with a knife edge 162 which coacts with a similar edge 163 on the tension member 160.

The particular construction of the switches 60 and 61 shown'here should not be construed as limiting the defrost system, since to one skilled in the art it may be possible to use other temperature-responsive devices, such as the use of heated wires which change length in response to temperature changes, and the material used in the switch shown or in other versions can be electrically heated by direct flow of electricity therethrough, rather than by separate heater elements. Also, other embodiments of circuitry having solid-state components could respond to the chill factor and cause the necessary defrost operation.

I claim:

1. A defrosting system for an appliance having a circulating airflow over cooling coils and means for supplying coolant to the coils comprising, selectively operable means for defrosting the appliance, means responsive to reduced airflow because of frost buildup on the cooling coils to cause operation-of said selectively operable means including a switch, an operating member for the switch which is constructed to move with temperature change positioned in said airflow and heated to a temperature above that of the airflow whereby the airflow cools said member to maintain said member in one position and with a predetermined reduction in airflow causing said member to move to operate said switch, and means for discontinuing operation of the defrosting means.

2. A defrosting system as defined in claim 1 wherein said means for discontinuing operation of the defrosting means includes a temperature-responsive switch having a normally closed contact in series with said airflow-responsive switch and switchable to a second position at said temperature level to establish normal oper ation of the appliance.

3. A defrosting system as defined in claim 4 wherein said temperature-responsive switch is responsive to a temperature below said temperature level to reengage the normally closed contact.

4. A defrosting system as defined in claim 1 wherein said operating member is constructed of bimetal and electrical means for heating thereof.

5. A defrosting circuit for a refrigerator or the like having cooling coils, a compressor and fan means for inducing an airflow over the coils, comprising defrosting means, a switch having a normally closed contact in series wth said compressor and fan means, an operating member for the switch constructed of material for movement in response to heat applied thereto and positioned in said airflow, means for applying heat to said operating member in an amount whereby normal airflow dissipates the heat sufficiently to prevent movement thereof and whereby a reduction in airflow permits the heat to shift the operating member and open said normally closed contact and energize said defrosting means.

6. A defrosting circuit as defined in claim 5 and including a temperature-sensing switch which bypasses said airflow-sensing switch to deenergize said defrosting means and condition said fan means and compressor for operation.

7. A defrosting system for an appliance having means providing a circulating airflow over cooling coils and means for supplying coolant to the coils comprising, selectively operable means for defrosting the appliance, a single pole double throw airflow-sensing switch in said appliance and in circuit with and normally in position to condition said circulating means and the coolam-supplying means for operation and including an operating member of material to change shape in response to temperature changes, means for heating said operating member, said operating member being normally cooled by the airflow and responsive to diminished airflow to shift to a second position by heat applied thereto to operate said defrosting means, and a second single pole double throw switch responsive to the temperature in the appliance and having a first position to place said first switch in circuit when the temperature in the appliance is at a low level and a second position to disable said first switch and condition said circulating means and coolant supply means for operation when the temperature in the appliance is at a higher level.

8. A system as defined in claim 7 wherein said operating member is of bimetal and is electrically heated.

9. A system as defined in claim 7 wherein said second switch has a wide differential temperature range and 

1. A defrosting system for an appliance having a circulating airflow over cooling coils and means for supplying coolant to the coils comprising, selectively operable means for defrosting the appliance, means responsive to reduced airflow because of frost buildup on the cooling coils to cause operation of said selectively operable means including a switch, an operating member for the switch which is constructed to move with temperature change positioned in said airflow and heated to a temperature above that of the airflow whereby the airflow cools said member to maintain said member in one position and with a predetermined reduction in airflow causing said member to move to operate said switch, and means for discontinuing operation of the defrosting means.
 2. A defrosting system as defined in claim 1 wherein said means for discontinuing operation of the defrosting means includes a temperature-responsive switch having a normally closed contact in series with said airflow-responsive switch and switchable to a second position at said temperature level to establish normal operation of the appliance.
 3. A defrosting system as defined in claim 4 wherein said temperature-responsive switch is responsive to a temperature below said temperature level to reengage the normally closed contact.
 4. A defrosting system as defined in claim 1 wherein said operating member is constructed of bimetal and electrical means for heating thereof.
 5. A defrosting circuit for a refrigerator or the like having cooling coils, a compressor and fan means for inducing an airflow over the coils, comprising defrosting means, a switch having a normally closed contact in series wth said compressor and fan means, an operating member for the switch constructed of material for movement in response to heat applied thereto and positioned in said airflow, means for applying heat to said operating member in an amount whereby normal airflow dissipates the heat sufficiently to prevent movement thereof and whereby a reduction in airflow permits the heat to shift the operating member and open said normally closed contact and energize said defrosting means.
 6. A defrosting circuit as defined in claim 5 and including a temperature-sensing switch which bypasses said airflow-sensing switch to deenergize said defrosting means and condition said fan means and compressor for operation.
 7. A defrosting system for an appliance having means providing a circulating airflow over cooling coils and means for supplying coolant to the coils comprising, selectively operable means for defrosting the appliance, a single pole double throw airflow-sensing switch in said appliance and in circuit with and normally in position to condition said circulating means and the coolant-supplying means for operation and including an operating member of material to change shape in response to temperature changes, means for heating said operating membEr, said operating member being normally cooled by the airflow and responsive to diminished airflow to shift to a second position by heat applied thereto to operate said defrosting means, and a second single pole double throw switch responsive to the temperature in the appliance and having a first position to place said first switch in circuit when the temperature in the appliance is at a low level and a second position to disable said first switch and condition said circulating means and coolant supply means for operation when the temperature in the appliance is at a higher level.
 8. A system as defined in claim 7 wherein said operating member is of bimetal and is electrically heated.
 9. A system as defined in claim 7 wherein said second switch has a wide differential temperature range and has a double-acting snap switch element with a bimetal operating member, and adjustable components associated with said switch element and bimetal operating member to have said switch element in one or the other of said positions. 